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Single Particle Versus Collectivity, Shapes of Exotic Nuclei

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Single Particle Versus Collectivity, Shapes of Exotic Nuclei EPJ manuscript No. (will be inserted by the editor) Single particle versus collectivity, shapes of exotic nuclei Andrea Jungclaus Instituto de Estructura de la Materia, CSIC, E-28006 Madrid, Spain Received: date / Revised version: date Abstract. In this article some selected topics of nuclear structure research will be discussed as illustration of the progress reached in this field during the last thirty years. These examples evidence the improvement of our understanding of the atomic nucleus reached on the basis of countless experiments, performed to study both exotic nuclei (nuclei far-off the valley of stability) as well as nuclei under exotic conditions (high excitation energy/temperature or large angular momentum/rotational frequency), using stable and radioactive ion beams. The experimental progress, in parallel to the advancement of modern theoretical descriptions, led us to a much richer view of this fundamental many-body system. PACS. PACS-key discribing text of that key { PACS-key discribing text of that key 1 Introduction During the last decades, nuclear structure physics has ex- perienced a worldwide renaissance and our understanding of the atomic nucleus has not only improved, but very of- ten we have found ourselves confronted with real surprises which demanded changes in our view of this fundamental system. As is often the case in the history of science, the very dynamic evolution of this field of research has been closely related to the development of new experimental tools. As we will see in the course of this article, the prob- ably most important new tools were on one hand side the first generation radioactive ion beams and on the other hand highly efficient 4π γ-ray spectrometer in conjunc- tion with different highly sophisticated ancillary detectors. In this article I will not try to provide an all-embracing Fig. 1. Chart of nuclei with the black squares marking the view of nuclear physics research but instead illustrate the about 300 stable nuclei existing in nature. The yellow area accomplished progress by means of some selected topics indicates the region of nuclei which already have been experi- (evidently chosen according to personal taste). The focus mentally studied while the green region consists of thousands of will be on the \simple physics behind" and the qualitative nuclei which are expected to be bound (stable against particle understanding rather then detailed technical or theoreti- emission) but have not yet been produced in terrestrial labora- cal aspects - assuming that many of these are presented tories. Note that many of these nuclei are however continuously in other contributions to this school. produced in the Universe during nucleosynthesis processes. The Fig. 1 shows the chart of nuclides, the orientation map path of the astrophysical rapid neutron capture process is in- for nuclear physicists. The black squares mark the about dicated in blue. Finally, the magic numbers of the nuclear shell 300 stable nuclei existing in nature and forming the valley model are included as black dashed lines. of stability. The nuclei on the left side of this valley decay via β+-decay, the ones on the right hand side via β−-decay. Since the pioneering work by Maria Goeppert-Mayer and excitation spectrum is dominated by single-particle exci- Hans Jensen in 1949 [1], we know that due to the shell tations, whereas nuclei with many protons and neutrons structure of the atomic nucleus, the latter is particularly outside the closed shells are very often deformed. In these stable (has a large binding energy) for certain numbers of nuclei collective excitations such as rotations or vibra- protons and neutrons, the so-called magic numbers. Nu- tions, to which many nucleons contribute, often become clei in the vicinity of closed shells are spherical and their energetically favoured. The basic predictions of both the spherical shell model and the collective models have been Send offprint requests to: confirmed in numerous studies in the past and it seemed 2 Andrea Jungclaus: Single particle versus collectivity, shapes of exotic nuclei that the atomic nucleus is - at least at not too high exci- nuclear physics textbook: i) the half-density radius of the 1=3 tation energy and angular momentum and not too far off matter distribution can be expressed as R = r0A with stability - very well understood. the radius constant r0 = 1:1 − 1:2fm, ii) protons and In the last decades, however, it became possible to ex- neutrons are uniformly mixed in the nucleus, or in other perimentally explore both exotic nuclei as well as nuclei words, there is no large difference observed in radii be- under exotic conditions. By exotic nuclei, we understand tween the proton and neutron distributions and iii) the nuclei far off stability, which includes i) nuclei with large surface thickness is constant. The question we are going isospin (T3=N-Z) close to the driplines and ii) superheavy to discuss now is whether these three common properties nuclei, i.e. nuclei with large mass number (A=N+Z). The of the nuclear density also hold for unstable nuclei. driplines mark the limits of nuclear stability, i.e. outside these borders the nuclei are unstable against the emission Of course, we cannot produce targets of unstable nu- of nucleons. In Section 2 of this article we will discuss how clei to perform scattering experiments with electrons or the first available energetic radioactive ion beams allowed protons. However, in the mid-eighties, first radioactive to study light neutron-rich nuclei and to map the neu- nuclear beams became available allowing for the deter- tron dripline up to Z=11 and what we have learned from mination of matter radii of the unstable nuclei from mea- these studies. After that we move along the neutron-rich sured cross sections for their interaction and reaction with side of the chart of nuclides to slightly heavier systems the nuclei of a stable target. In the pioneering work by and treat the evolution of nuclear shell structure at large Tanihata and co-workers [2], which was performed at the isospin. Then we will discuss recent progress in the study Lawrence Berkeley Laboratory (USA), the interaction cross of the properties of nuclei in the region around doubly- sections between radioactive Li and Be beams and stable magic 132Sn and illustrate the relevance of the new results Be, C, and Al targets have been measured. For the first in view of the astrophysical rapid neutron capture process time, a sudden increase of the square mean radius has (r process) of nucleosynthesis. In Section 3 we move to been observed within the Li chain of isotopes, in partic- the neutron-deficient side of the valley of stability and ular between 9Li and the next bound isotope 11Li. This discuss topics such as the isospin symmetry in medium- observation has been interpreted soon later as first evi- mass N=Z nuclei, the contribution of T =0 isoscalar pair- dence of a neutron halo in 11Li by Hansen and Johnson ing to the properties of heavy N=Z nuclei, the proton [3]. Meanwhile, nuclei radii have been measured for nu- emission in nuclei close to, and in some cases even across, merous light unstable neutron-rich nuclei using the same the proton dripline and finally the phenomenon of shape technique. The results are summarized in a schematic way coexistence. To close the discussion of nuclei close to the in Fig. 2. It is immediately obvious from this figure that 1=3 limits of stability we will discuss in Section 4 the status the R = r0A rule is not valid at all far off stability, of superheavy nucleus synthesis and close with remarks at least in the region of light nuclei. In contrary, large about the spectroscopy of transfermium nuclei. variations in radii are observed. Marked in this figure are Complementary to the investigation of exotic nuclei is the cases of well established 1n and 2n halos and the so the study of nuclei closer to stability, but under exotic far only known case of a proton halo in 8B. Whereas in conditions. In the last section of this article we will try the case of 11Li the radius changes drastically from one to answer questions such as: How does a nucleus behave isotope to the next, a gradual increase of the matter ra- at high angular momentum and/or high excitation ener- dius is observed in the chains of heavier F, Ne and Na gies? Can a collapse of pairing correlations be observed isotopes. This phenomenon is understood as a neutron under these circumstances? How does the shape of a nu- skin, which is an excess of neutrons at the nuclear sur- cleus change with angular momentum? What is the limit face. Generally it is not trivial to determine the radii of of deformation a nucleus can sustain? Finally, the arti- the proton and neutron distributions separately. An esti- cle closes with a short outlook on the future of nuclear mate of the charge radius can be obtained from the cross structure research. sections for charge-changing reactions at relativistic ener- gies. However, a unique opportunity for a precise compar- ison between matter and charge radii over a wide range 2 The neutron-rich side of the nuclear chart of neutron numbers is offered by the Na isotopes, since their rms charge radii have been determined in isotope- 2.1 Halos, skins and clusters in light nuclei shift measurements [4]. The matter radii were determined as described above [5] and the resulting proton and neu- The nuclear size and the proton and neutron density dis- tron radii are shown in Fig. 2. We can see a gradual growth tributions are important bulk properties of nuclei that de- of the thickness of the neutron skin for neutron-rich Na termine the nuclear potential, single-particle orbitals, and isotopes up to 0.4 fm.
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